1. Field of the Invention
The present invention relates to a screen process printing method of performing solid printing with a printing ink on a printing substrate such as a paper sheet, a film or a metallic foil, using a screen pattern plate through which the printing ink is brushed or squeezed onto the printing substrate by a squeegee, and a screen printing machine for performing the screen process printing method, which can be used for the sublimation type ink ribbon, batteries, and scratch cards.
2. Discussion of Background
Conventionally, when a screen process printing method is employed in the production of color ribbons such as a thermosensitive recording ribbon and a sublimation type ink ribbons, a printing ink is applied to a screen pattern plate which include opening portions in the same pattern as a solid image pattern to be printed as shown in FIG. 4 and is brushed or squeezed through the screen pattern plate onto a printing substrate such as a paper sheet, a film or a metallic foil which comes into contact with the screen pattern plate, using a squeegee which is brought into pressure contact with the screen pattern plate, whereby a solid printed pattern is obtained as desired.
Usually, the printing ink for use in color printers for the production of thermosensitive recording ribbons and sublimation type ink ribbons has a high viscosity in a range of 5,000 to 20,000 mPa.s, so than when the printing speed (that is, the transportation speed of the printing substrate such as a paper sheet or a film) is increased, printing defects are caused in printed image areas, such as lack of surface smoothness in the printed image area with an uneven or wavy printed surface, the trailing of rear end portions of printed images, the smearing of printed areas, due to the filamenting and misting of the printing ink.
In the case of printing ribbons which are produced by the screen process printing method, using the printing ink, the lack of surface smoothness would directly cause the lowering of the printed image quality that can be obtained by the printing ribbons, so that high smoothness is required and demanded for the surface of such printing ribbons. In the case of color printing ribbons, printing is continuously performed using three or four yellow, cyan, magenta and/or black rectangular patterns. However, the above-mentioned problems such as the filamenting and the misting of the printing ink must be avoided, since such problems would cause the smearing of adjacent color patterns and the above-mentioned printed image defects.
The mechanisms of the occurrence of the filamenting and the misting of the printing ink have not yet been clarified. However, it is considered that the filamenting of the printing ink is caused by forming a thread with the aggregation of several ink droplets which pass through ink passing holes or openings of the screen pattern plate, or by the formation and growth of bubbles in the printing ink with a reduction in an inner pressure of the printing ink immediately before the breakup or fragmentation of the printing ink, and a successive non-uniform reduction in the inner pressure of the printing ink in the direction of the width of the screen plate, and that the filamenting of the printing ink causes the misting of the printing ink.
In order to prevent the occurrence of the filamenting and the misting of the printing ink, the humidity of the ambient printing atmosphere is empirically controlled, and conductive materials for increasing the conductivity of the printing ink and other various compounds are added to the printing ink.
However, in the case of thermosensitive recording ribbons, sublimation type ink ribbons, and batteries, each printed area is required to have its own particular functional quality, so that in many cases, various compounds cannot be added to the printing ink, and therefore the occurrence of the filamenting and the misting of the printing ink is prevented inevitably, for instance, by reducing the printing speed.
However, this is not an efficient method, in particular, for producing a thermosensitive recording ribbon, a sublimation type ink ribbon, batteries, and scratch cards.